Recently, considerable attention and efforts have been directed to the reduction of nitrogen oxides resulting from the combustion of fuel. This is especially true in the area of large furnaces or boilers such as used by the power generation utilities which utilize coal as their main fuel source. In a typical arrangement for burning coal in a large boiler, several burners are disposed in communication with the interior of the boiler and operate to burn a mixture of air and pulverized coal. The burners used in these arrangements are generally of the type in which a fuel-air mixture is continuously injected through a nozzle so as to form a single, relatively large flame. As a result, the surface area of the flame is relatively small in comparison to its volume, and therefore, the average flame temperature is relatively high. However, in the burning of coal, nitrogen oxides are formed due to the reaction of nitrogen present in the combustion-supporting air with oxygen. The formation of nitrous oxides is a function of flame temperature. When the flame temperature exceeds 2800.degree. F., the amount of nitrogen removed from the combustion-supporting air rises exponentially with increases in the temperature. This condition leads to the production of high levels of nitrogen oxides in the final combustion products, which is undesirable.
Nitrogen oxides are also formed from the fuel bound nitrogen available in the fuel itself, which is not a direct function of the flame temperature, but is related to the quantity of available oxygen during the combustion process.
Low Nox burners for use in power or utility boilers are now available for certain types of furnaces. For example, U.S. Pat. No. 5,249,535 illustrates an example of such a burner. The burner disclosed in this U.S. patent, however, is not suitable for use in a tangentially-fired furnace. In a tangentially-fired furnace, burners are usually located at each corner of a square-shaped combustion chamber. The axes of the burners are offset with respect to a central axis of the combustion chamber and are generally tangent to an imaginary cylinder which defines a combustion zone where a fireball is generated during operation of the burners. Since many utility boilers are heated by a tangentially-fired furnace, it is desirable to provide a burner capable of low nox emissions for these types of furnaces.